A phosphor is utilized in a fluorescent display tube (VFD), a field emission display (EFD), a plasma display panel (PDP), a cathode-ray tube (CRT) a white color light-emitting diode (LED), and so on. In any of these applications, it is necessary to provide the phosphor with energy to excite the phosphor in order to render the emission and the phosphor is excited by the excitation source with high energy such as a vacuum ultraviolet ray, an ultraviolet ray, an electron beam, a blue light, and the like so as to emit a visible light. However, the phosphor deteriorates as a result of exposure to the above-mentioned excitation source such that there is an issue that the brightness of the phosphor decreases as it is used for a long period of time and the phosphor in which the brightness does not deteriorate is desired.
A white color LED has been used in a field of disaster light, beacon, and the like where reliability is required, a field of in-vehicle light, a back light of a mobile phone, and the like where reduction in size and weight is strongly desired, a field of direction board where visibility is required. The emission color of this white color LED, i.e., the white light may be obtained by mixing lights such that the white light is a mixture of a yellow light emitted by a phosphor and a blue light emitted by a blue color LED of wavelength from 430 to 480 nm as the emission source. The phosphor suitable for such white color LED is, by a small amount, arranged on the surface of the blue color LED chip as the emission source. Therefore, the phosphor which emits a yellow light upon irradiation of the blue color LED is desired for this application. Further, in view of reducing fluctuation of emission color caused by the temperature change of the application environment where the device is used, phosphor material emitting fluorescence with a small fluctuation in the emission intensity caused by the temperature change is also desired.
As the emission material to emit a yellow light upon irradiation of the blue color LED, garnet ((Y, Gd)3(Al, Ga)5O12: Ce, hereinafter, referred to as “YAG: Ce”), which is an oxide, is known. This phosphor was formed by replacing Y sites partially with Gd and Al sites partially with Ga, and doping Ce3+ as the optically-activating ion at the same time (Non-patent reference 1). Although this phosphor is known as a highly efficient phosphor, the emission intensity is lowered as the temperature increases such that there is an issue that the emission color of the device varies depending on the temperature when it is used in the white color LED.
A phosphor comprising α-type sialon as a host crystal is proposed as a yellow color phosphor with a small temperature fluctuation of emission. The α-type sialon is a crystal to form an interstitial solid solution wherein Li, Ca, Mg, Y, or a lanthanide metal solid solves interstitially into the α-type Si3N4 crystal. The α-type Si3N4 crystal structure has two large spaces having diameters of about 0.1 nm interstitially in the unit cell. The structure is stabilized if metal atoms solid solve in such spaces. Therefore, the general formula of α-type sialon containing a metal element M is given by:Mx(Si12−(m+n)Alm+n)(OnN16−n).Here, x is the number of M atoms contained in the α-type Si3N4 unit cell. Further, m corresponds to the number of Al—N bonds substituting Si—N bonds in the α-type Si3N4 crystal structure and m=δx (here, δ is the valence number of metal M). Here, n is the number of Al—O bonds substituting Si—N bonds. The electric neutrality is maintained by the above lattice substitution and interstitial solid solution. In the α-sialon, metal-nitrogen bonds are main bonds such that the α-sialon is a solid solution with a high percentage of nitrogen content.
It is publicly know before this patent application that α-type sialon becomes a phosphor if some of stabilizing metal atoms solid solving interstitially into the α-type sialon are substituted with optically activating metal ions (Non-patent references 2 to 4). One (Mr. Mitomo) of the present inventors found a phosphor material having Ca-α-sialon as a host crystal and being doped with Eu2+ became a material to render a yellow color emission upon irradiation of a visible light of the violet-blue wavelength region (Patent references 1 and 2).
It is disclosed that this material emits a yellow light which is a complementary color of a blue color upon irradiation of the excitation light of the blue color LED, and that this material can be used as a phosphor for the white color LED by mixing lights of both colors (Patent reference 3). However, in these materials, there still is an issue that the emission intensity is not high enough because the amount of Eu2+ solid solving into the α-type sialon lattice is small. Further, it was reported that Ca-α-sialon doped with Eu became a phosphor to emit a yellow light of 550 to 600 nm upon excitation by the blue light of 450 to 500 nm. However, in the composition having the best emission efficiency, the emission wavelength is from 580 to 600 nm such that a white color LED having the excitation source of the blue color LED emitting a light of 450 to 470 nm emits a white light with mixed colors which is a lamp color to have the correlated color temperature of 3000 K. Therefore, it was difficult to obtain the light emission of a daylight color, a day white color, and a white color of the correlated color temperature of 5000 K to 6500 K, which is usually used for an ordinary light.
The researches for adjusting solid solution metal and solid solution amount in the α-type sialon contained in the phosphor as the host crystal are conducted (Patent reference 4). Although Li-α-sialon doped with Eu2+ is also disclosed in some of the researches, the emission intensity was not high enough for the practical use. It is inferred that this is because only a little amount of added Li and Eu were solid-solved in the lattice. Since the emission wavelength of Li-α-sialon phosphor in which Eu is solid-solved as described in Patent reference 4 is 585 nm and the phosphor and a blue LED may be combined together to produce a light bulb color, a phosphor with the emission wavelength of 580 nm or shorter was desired in order to produce a white color (chromaticity: x=0.33 and y=0.33). That is, in the α-type sialon with an Eu luminescence center, a yellow-green color phosphor rendering the emission of shorter wavelength was desired.
In the conventional technology of the lighting apparatus, a white color light-emitting diode of a combination of a blue color light-emitting diode device and a yellow color emission phosphor to be excited by the blue color is publicly known and is implemented in various kinds of lighting applications. As typical examples thereof, Japanese patent No. 2900928, “Light-emitting diode” (Patent reference 5); Japanese patent No. 2927279, “Light-emitting diode” (Patent reference 6); Japanese patent No. 3364229, “Wavelength conversion material and its manufacture and light emitting device” (Patent reference 7); and so on are cited. In these light-emitting diodes, phosphors being used particularly often are phosphors in the Ce-activated yttrium-aluminum-garnet (YAG:Ce) system and expressed by the general formula: (Y, Gd)3 (Al, Ga)5O12:Ce3+.
However, the white color light-emitting diode comprising: a blue color light-emitting diode device and a phosphor in the YAG:Ce system has the emission intensity which is lowered when the temperature increases such that there was an issue that the emission color fluctuated because of deteriorated balance between the blue and yellow lights when the devices were warmed up as the time goes by after turning on the switch.
In these backgrounds, a phosphor which emits a yellow-green light of shorter wavelength than that of Ca-α-sialon doped with Eu and shows the brightness of smaller temperature fluctuation than the phosphor in the YAG: Ce system was desired.
[Patent reference 1] Japanese patent application publication No. 2002-363554
[Patent reference 2] Japanese patent application publication No. 2003-336059
[Patent reference 3] Japanese patent application publication No. 2004-186278
[Patent reference 4] Japanese patent application publication No. 2004-67837
[Patent reference 5] Japanese patent No. 2900928
[Patent reference 6] Japanese patent No. 2927279
[Patent reference 7] Japanese patent No. 3364229
[Non-patent reference 1] Mukai, Nakamura, “White and UV LEDs,” OYO BUTUR1, Vol. 68, 152-55 (1998).
[Non-patent reference 2] J. W. H. van Krevel, “On new rare-earth doped M-Si—Al—O—N materials luminescence properties and oxidation resistance,” Thesis, ISBN 90-386-2711-4, Eindhoven Technische Universiteit Eindhoven (2000).
[Non-patent reference 3] J. W. H. van Krevel et al. “Long wavelength Ca3+ emission in Y—Si—O—N materials,” J. Alloys and Compounds, 268, 272-277 (1998))
[Non-patent reference 4] J. W. H. van Krevel et al, “Luminescence properties of terbium-, cerium-, or europium-doped α-sialon materials,” J. Solid State Chem. 165, 19-24 (2002).
[Non-patent reference 5] R. J. Xie et al, “Preparation and Luminescence spectra of calcium- and rare-earth (R=Eu, Tb and Pr) codoped α-SiAlON ceramics,” J. Am. Ceram. Soc. 85, 1229-1234 (2002).